A thin film composite (TFC) membrane is a membrane that has layers of dissimilar materials joined together to form a single membrane. This layered construction permits the use of material combinations that optimize performance and durability of the membrane.
TFC membranes are used in nanofiltration, reverse osmosis (RO) and forward osmosis membranes. Such membranes typically are made by interfacial polymerization of a monomer in a nonpolar (e.g., organic) phase together with a monomer in a polar (e.g., aqueous) phase on a porous support membrane. TFC membranes are used where flux and substantial rejection characteristics are required, e.g., in the purification of water. Various materials and chemical additives have been added during formation of the discrimination layer of the membranes to increase flux without reducing rejection characteristics. For example, U.S. Pat. No. 6,024,873 describes adding alcohols or ethers to the aqueous or organic phase before interfacial polycondensation to form the discrimination layer in order to increase water flux of the membrane. U.S. Pat. No. 8,177,978 teaches that diketonates and metal additives can be added to the aqueous or organic phases before interfacial polycondensation to form the discrimination layer in order to increase water flux. Other additives are described in U.S. Pat. Nos. 8,567,612; 8,801,935; and 8,029,857.
There are methods that purportedly improve the water flux of membranes by treating the membrane after formation of the discrimination layer. For example, U.S. Pat. No. 4,888,116 describes treating reverse osmosis membrane having a polyamide discrimination layer with an aqueous solution of a reagent that reacts with primary amine groups to form diazonium salt groups or derivatives of diazonium salt groups, which can increase the water flux of the polyamide membrane with purportedly little or no effect on the salt rejection of the membrane. U.S. Pat. No. 3,551,331 describes a process for modifying the permeability of a polyamide membrane by treatment with a protonic acid, lyotropic salt or a Lewis acid. U.S. Pat. No. 3,904,519 describes treatment of linear aromatic polyamides with crosslinking reagents to improve flux or flux stability of the resulting membranes.
The treatment of TFC membranes with chlorine or a chlorine generating agent to enhance their performance is disclosed in a number of patents. U.S. Pat. No. 4,277,344 describes the post-treatment of a polyamide membrane with a solution containing 100 ppm hypochlorite for one day. U.S. Pat. No. 4,761,234 describes the treatment of a polyamide TFC membrane that includes a triamino-benzene as one monomer with an aqueous solution containing 1000 ppm residual chlorine at a pH of 10.3 at room temperature for 18 hours. U.S. Pat. No. 5,051,178 describes the treatment of certain types of polyamide TFC membranes obtained by interfacial polymerization involving mixtures of triamines with 100-600 ppm chlorine or a chlorine generating agent at a pH of 6-13 for a time period of from 2 minutes to 45 hours. These patents demonstrate that treatment of a membrane with an agent releasing chlorine may improve the water flux of a polyamide membrane, but the improvement in water flux is generally at the cost of some other performance characteristic of the membrane, such as stability or salt rejection. U.S. Pat. No. 5,876,602 purportedly addresses these shortcomings by treating a composite polyamide reverse osmosis membrane with hypochlorite ion at a concentration of 200 to 10,000 ppm at a pH of at least 10.5 and an exposure time of from 30 minutes to 3 hours to improve water flux and salt rejection, as well as to increase the stability to alkaline conditions.
Some of the suggested methods in the art are too cost prohibitive to implement commercially. In others, the reactions can be difficult to control under production conditions, or can damage the membrane or processing equipment, or the improvements in the membrane can be transitory, or the treatments can result in increased water flux while sacrificing salt rejection over time.
Although many commercial polyamide membranes can display an excellent combination of water flux, salt rejection and chemical stability, membranes having improved performance characteristics are still being actively sought. What are needed are additional techniques for treating the membrane after formation of the discrimination layer that result in improved water flux while maintaining or improving rejection characteristics of TFC membranes.